1. Field of the Invention
The invention relates to a method of Nuclear Magnetic Resonance imaging, which method includes the following steps:
a) generating a static magnetic field having appropriate intensity and homogeneity characteristics in a predetermined imaging volume;
b) introducing a body or a part of said body in said imaging volume for examination;
c) generating magnetic field gradients in several different spatial directions to select the scan slice of the body under examination and to univocally phase-encode nuclear spins in the selected slice;
d) generating sequences of electromagnetic signals to excite the nuclear spins of the body or of the body part introduced in the imaging volume;
e) receiving the resonance signals emitted by nuclear spins and processing them to extract information therefrom and reconstruct the corresponding images which images relates only to a predetermined limited region of the body or of a part thereof to be examined;
f) displacing the body or the part thereof to be examined to a predetermined extent from said first imaging position to at least a second imaging position, the above steps being repeated for further regions of the body part under examination corresponding to further relative positions between the body or part thereof and the imaging volume, by displacing the body or the part thereof in predetermined directions relative to the imaging volume or vice versa.
2. Discussion of Related Arts
These methods are currently well-known and widely used. In most prior art systems imaging volumes are generated, i.e. regions of space permeated by a static magnetic field with the characteristics required for proper imaging, particularly intended for diagnostic purposes. This is aimed at obtaining considerably large imaging volumes, to allow imaging of relatively wide regions of the body under examination, particularly to find the region of the body part which is relevant for diagnostic purposes. The provision of these comparatively large imaging volumes requires apparatuses having large-sized magnetic structures. In addition to their high purchase cost, these apparatuses are expensive to install and need to be located in appropriate facilities.
EP 654 675 discloses an apparatus having a simplified xe2x80x9ctotal bodyxe2x80x9d construction for Nuclear Magnetic Resonance tomography. According to this document, a magnetic field is generated for Nuclear Magnetic Resonance imaging, which permeates a substantially cylindrical volume whose axial dimension equals from 10 to 15 cm. The body under examination is displaced in the direction of the cylindrical body axis, to allow imaging of successive slices in different regions of the body. The processed images or received signals are stored sequentially and in relation to the scan slice of the body wherefrom they were generated to provide a virtual three-dimensional image in the memory. In order to limit scanning time, particular and relatively fast sequences are used, such as those named echo-planar sequences. During the scanning operation, the information relating to the whole scanning cross section of the body is detected, processed and stored. Once the data obtained by scanning the successive slices are stored, they may be recalled and displayed. The construction of the apparatus derives from that of the so-called total-body apparatuses and, with respect to the latter, the ring-like magnetic structure is only axially shortened depending on whether the volume permeated by the imaging static field is selected as being an axially short cylinder or a thin disk.
EP 430 222 discloses a method for Nuclear Magnetic Resonance tomography imaging of the same type as radiological scanograms. Here again the magnetic structure derives from xe2x80x9ctotal bodyxe2x80x9d constructions, i.e. those designed for imaging the whole body or a considerable part thereof. The method disclosed in this document equally provides that a succession of different slices of the body under examination are scanned, by displacing the body under examination relative to the useful volume permeated by the static magnetic field. However, unlike the method of EP 654 675, this method provides the selection of a slice plane parallel to the longitudinal extension of the body under examination and parallel to the displacement direction of said body, by applying dephasing sequences, e.g. for saturating the nuclear spins of each cross section beyond the slice oriented in the direction of advance of the body, to limit the detection of echo signals to those actually emitted by the spins of the slice and/or section of the body oriented in the displacement direction. Here again the images are processed and generated at the end of the whole scanning process.
Hence, according to both prior art methods, images are displayed after the whole body or region of interest thereof have been scanned.
Now apparatuses are known, in which the magnetic structure is drastically limited in size, with a consequent reduction of the volume in which the magnetic field has the characteristics required for proper imaging. These apparatuses provide considerable advantages in terms of money savings, but do not allow to obtain relatively wide images of the relevant region, or not at a high quality level.
Moreover, in these apparatuses the problem exists of finding the region of the body which is of relevance or of diagnostic interest. Hence, repeated scanning operations of different regions of a body part are required to frame the region of diagnostic interest. For instance, for a diagnosis on a meniscus, several different scans of the knee may be required to obtain the proper, best shot of the meniscus region.
These scans are currently made with sequences adapted to provide the best image in terms of definition and contrast, but these sequences are comparatively long and complex, so much longer scanning times are requested. Even in total body apparatuses, or the like, in which the structure allows for relatively wide imaging volumes, the sequences are adapted to ensure the best image quality, resulting in a long scanning and processing time.
Moreover, even in the so-called xe2x80x9ctotal bodyxe2x80x9d apparatuses, the quality of the image might not be consistent all over the volume and anyway imaging time tends to be comparatively long when obtaining wide images.
Imaging sequences for generating high quality images, besides being time-consuming, also involve considerable costs as regards the construction of the apparatus, which has to be configured to execute them within acceptable times.
Prior art apparatuses have such characteristics that they do not allow a real time, or almost real time display of the detected images, moment by moment, e.g. while the body under examination is displaced. This is a drawback when imaging is performed in transient conditions of the body under examination, which may be either pathologic or physiologic or induced by relatively invasive techniques, such as the use of substances, injected into the body under examination, designed to improve the signals received from the body under examination or parts thereof, such as contrast agents for Nuclear Magnetic Resonance.
The invention is based on the acknowledgement that, in practice, no excessively wide image of the body, limb, or region thereof is required for the purpose of finding the specific shot for diagnosis or for a specific examination and for finding the right acquisition moment with reference to induced transient conditions. Images of wide portions of the body, the so-called panoramic images are used to provide an overall view of the situation. Certain xe2x80x9ctotal bodyxe2x80x9d apparatuses use sequences which allow almost real time imaging upon introduction of the patient in the imaging cavity, i.e. as the patient gradually enters the magnet cavity. These techniques are highly complex and require heavy and fast processing, which may be only performed in advanced and costly hardware structures, contained both in the NMR apparatus and in the processing unit.
The invention has an object of providing a method for Nuclear Magnetic Resonance imaging as described hereinbefore, which allows fast and easy location of the best shot of the relevant regions of the body, while providing both high quality images of very restricted regions of interest and panoramic images of portions of the body to be inspected, for example, for said regions of interest, the whole with relatively simple and inexpensive means, as compared with current costs of Nuclear Magnetic Resonance apparatuses, and regardless of the type and size or purpose of the apparatus.
The invention achieves the above purposes by providing a method as described above, in which:
h) the useful imaging volume permeated by the magnetic field, having sufficient characteristics to ensure proper Nuclear Magnetic Resonance imaging, is limited in all spatial directions with reference to the volume of the body under examination or part thereof, the body under examination being displaced relative to said volume in the three spatial directions.
By this arrangement, even without editing the scanning sequences, the reduction of the useful imaging volume limits the amount of information to be processed thereby reducing processing and displaying times.
In accordance with an additional characteristic, the reduction of the useful volume and/or the choice of the definition, i.e. the number of lines or pixels to be processed for image display and/or the characteristics of the scanning sequences which determine the signal-to-noise ratio and/or contrast are set separately or in combination in such a manner as to limit detection and/or processing and/or display times for each image to such an extent as to obtain total detection, processing and display times for the scanned image below one second, preferably a fraction of a second.
In this manner, the scanned image may be displayed with an acceptable delay, with respect to the moment in which the new relative position between the imaging volume and the body under examination is reached.
Typically, a total time for the steps of exciting echo signals, receiving and processing and reconstructing, as well as displaying the image of about {fraction (1/10)} of a second to a second allows to display the different images showing the different displacement conditions, by using standard sequences, not heavy as regards hardware equipment and construction. Therefore, unlike prior art methods, particularly those disclosed in the above documents, any displacement of the body under examination causes an immediate or substantially immediate display of the relevant scanned image.
With reference to an additional characteristic of the method according to this invention, there is particularly provided the possibility to reduce, by saturation or dephasing sequences, the imaging volume, i.e. the so-called FOV (Field of View), as compared with the largest size thereof provided by the magnet structure. Such reduction may be calibrated with respect to the size of the particular region of interest of the body under examination.
Hence, for instance, for a diagnostic examination on the vertebral column or, for instance, of a meniscus, the imaging volume, i.e. the Field of View may be reduced to a size which is slightly larger or smaller than the region of particular interest of the body under examination.
Typically, in an examination of the vertebral column or the meniscus, the FOV might be reduced to a few centimeters, typically to a sphere or a disk having a diameter of a few centimeters, particularly of about 5 cm. In addition to the reduction of the signal to be processed, hence of the total imaging times from excitation of the matter to image display, the advantage is provided that said reduction allows to operate in the central portion of the imaging volume, farther from the periphery of the magnetic structure, in which the magnetic field is distorted in such a manner as to cause image quality losses.
Moreover, it has to be noted that, while a drastic reduction of the total imaging time is achieved with an equal resolution, i.e. equal number of lines or pixels, the drastic reduction of the FOV allows to increase resolution and to keep such total time at the same level as an imaging process with a larger FOV and a lower resolution.
An additional characteristic of the method of the invention allows real time editing of the imaging sequence/s, with reference to the type of said sequences, to the variation of the field of view (FOV) and/or to their duration and/or to additional parameters to effect a change aimed either at reducing or at improving image quality, with reference to definition and/or to signal-to-noise ratio and/or to contrast and/or to the presence of artifacts and to immediately display the image obtained thereby with the new features. In this case, for instance during body displacement and fast scanning of images for the different positions therefore, and immediate display of said images, the user may stop the relative displacement of the body under examination or of the magnetic structure and perform higher quality imaging on the set position to verify if the region whereof the image has been scanned is already the one of interest or relevance for the examination. If this is not the case, the user may restore the starting conditions and continue the examination with the relative displacement between the body under examination and the magnetic structure.
Alternatively, the total imaging time from echo excitation to image display may be modified to take advantage of higher or lower frame rates, i.e. the speed at which the individual images of the different slices are displayed. This is advantageous in that, in regions of the body under examination that are far or assumed to be far from the region of interest, the quality of the image may be limited to increase the display speed, i.e. to reduce the imaging time from echo excitation to image display in the region of lower or poorer interest, whereas imaging times may be extended in the regions of higher interest, while keeping constant or substantially constant total examination times, though obtaining images with a better quality for the regions of higher interest of the body under examination.
Particularly, in combination with typical magnetic structures of the so-called dedicated apparatuses, i.e. having cavities of the magnetic structure for housing only a portion of the body under examination and especially with C-shaped magnetic structure, there may be provided an optical visual indication of the slice whereto the image pertains, which may be based either on the specific morphology of the displayed regions of the body under examination or on optical indicators or pointers directed on the body under examination. The method of the invention also includes the steps of univocally defining the relative position between the body or part thereof and the imaging volume whereto each detected image pertains, while the parameters of this displacement, i.e. essentially the direction and magnitude of the displacement vector are stored in relation to the data of the corresponding scanned images. Hence, during the examination, i.e. when the successive images of the different regions of the body are scanned, a virtual three-dimensional image may be generated, stored and recalled, to allow a future reconstruction or display of the images of the different slices, or o a three-dimensional image.
According to an improvement, the individual images of the predetermined regions of the body under examination or of the part thereof are obtained with scanning sequences intended to provide low definition images.
As previously stated, once a predetermined shot, or the proper desired shot of the region of interest of the body or body part has been defined, the method of the invention provides the possibility to perform acquisition scanning with sequences intended to provide alternatively and/or in combination a higher definition and/or a higher contrast and/or a better signal-to-noise ratio.
Besides the possibility to edit scanning sequences and modes for signal processing, transformation into images and filtering said images, each of the image characteristics which are to be adjusted, i.e. particularly definition, contrast and signal-to-noise ratio may be adjusted separately.
The invention advantageously provides a separate control for the characteristics of the acquisition sequences and/or of the processing modes and/or of image filters with reference to the effect on images particularly as regards definition, contrast and signal-to-noise ratio. The control for adjusting image quality in terms of definition and/or contrast and/or signal-to-noise ratio may operate in a continuous or step-like manner, as determined by the continuous or discrete variability of the parameters of sequences and of processing software or by their consisting of additional sequence or processing steps or of a combination of sequences or processing procedures or of repeated sequences or processing procedures.
In one embodiment, there may be provided a certain number of stored basic scanning sequences and/or image processing procedures, which may be combined with each other, the control for adjusting image definition and/or contrast and/or signal-to-noise ratio characteristics consisting of controls for selecting and/or combining said sequences and/or said processing procedures. There may be also provided predetermined combinations of basic sequences or basic procedures, which are related to a change of definition and/or contrast and/or signal-to-noise ratio, the corresponding change controls simply consisting of selections between predetermined and preset combinations. In this case, each combination provided is associated to an indication for the user, which does not define the combination of sequences or processing procedures, but the related level of definition and/or contrast and/or signal-to-noise ratio.
In accordance with an additional characteristic of the invention, the different shots of partial regions of the body or of the body part under examination, which are imaged in the same slice plane, but in regions other than the slice of the body under examination and related to their relative position between the body or body part and the imaging volume, may be combined together to form a panoramic image composed of the individual partial images of the body or of the part thereof.
In this case, low definition, low contrast and low signal-to-noise ratio panoramic images may be obtained, hence low or medium quality panoramic images. It is also possible to obtain high quality panoramic images, separately and/or in combination, in terms of definition, contrast and signal-to-noise ratio, the user being allowed the possibility to operate on the characteristics of imaging sequences and/or on image processing and/or filtering modes in such a manner as to change, separately or in combination, definition and/or contrast and/or signal-to-noise ratio.
Hybrid panoramic images may be also obtained, i.e. composed of partial shots, each having at least partly an inconsistent image quality, particularly in terms of definition and/or contrast and/or signal-to-noise ratio. The relative displacement of the body or a part thereof and of the imaging volume may also follow one, two or all three spatial coordinates.
This displacement may account for the size and morphology of the predetermined limited region whereof imaging is performed, so as to obtain images of limited regions whose boundaries are substantially coincident, slightly overlapping or spaced from each other.
Different extensions, i.e. different Fields of View (FOV) might as well be provided for each limited area, depending on the relevant anatomic detail.
Particularly, when obtaining panoramic images in which the element of interest is a portion of the panoramic image composed of individual images of adjacent or contiguous limited regions, peripheral regions may be imaged with reference to larger predetermined limited regions as compared with the predetermined limited regions containing the elements of interest, whereby there may be provided partial shots or partial images having different width (FOVs) or displaying partial regions of the body or of a part thereof having different widths.
Particularly, when Nuclear Magnetic Resonance images of the individual limited regions are sectional images of the body or of a part thereof, the above described technique not only provides two-dimensional panoramic images, but, like in three-dimensional graphics, the successive images of the limited regions may be related together to compose, combine or reconstruct even three-dimensional images of the region of interest and with more or less panoramic characteristics.
The displacement may be performed in steps of equal width or increasing or progressing according to a predetermined rule, said steps being for instance smaller in the region of highest interest of the part under examination and larger in the region of lowest interest. In this case, the individual images obtained will be images of partial regions which are closer to each other in the region of interest and reduce their nearness in the regions surrounding the region of interest or examination of the body or of the part thereof.
The advantages of method of the present invention are apparent from the above disclosure.
First, the method according to the invention allows to find the best shot for instance for the purpose of diagnosis, without requiring excessively long imaging times or heavy construction changes and integrations in the structure of the NMR apparatus and in the processing units. Conversely, the advantage that the shot of interest typically relates to a relatively small region of the body or of a part thereof might be used to restrict the construction size and complexity of the apparatus and to reduce the imaging volume, thereby reducing scanning and image processing times.
The image of the newly imaged slice may be displayed directly and in real time with a certain delay with respect to the displacement of the body under examination, whereby a direct visual control is possible both on the image displayed and on the morphological position in a certain portion of the scanned and displayed position.
The additional arrangement of allowing the independent user selection, either separately or in combination, of the adjustments in scanning sequences and in image processing procedures and the imaging process allows the user to obtain an image having the most appropriate quality for the purpose, for instance, of image evaluation.
Also, the method according to the invention allows the formation of Nuclear Magnetic Resonance panoramic images even when using dedicated apparatuses, or apparatuses with very small magnetic structures, which are intrinsically able to generate or ensure limited imaging volumes. This provides a considerable economic advantage in terms of purchase as well as installation costs and important advantages in terms of convenience and functionality of use of these apparatuses by users.
The method according to the invention provides advantages in apparatuses which allow the generation of relatively large imaging volumes and hence of relatively wide images. In fact, zoom techniques aimed at FOV reduction, thanks to which smaller images of regions of the body or of body parts may be obtained as compared with normal imaging conditions allow to limit scanning, processing and display times. A combination of zoom techniques with the method according to the invention allows, in this case, to obtain partial images consisting of combinations of images obtained with the zoom technique and showing adjacent regions of the body or of the body part, or three-dimensional images showing a region of the body or of the body part which is smaller than the one usually permeated by the imaging volume.
The above description also clearly shows the versatility of the method according to the present invention.
The invention also relates to an apparatus for Nuclear Magnetic Resonance imaging, which apparatus includes:
a) a magnetic structure for generating a static field in a predetermined volume inside an imaging cavity and with at least one opening to provide access to said imaging cavity;
b) at least one transmitting coil and at least one receiving coil;
c) at least one, preferably more gradient coils;
d) means for processing resonance signals to transform them into displayable image signals;
e) means for displaying image signals; The apparatus according to the invention additionally includes, to implement said method:
f) means for relative displacement of the body under examination and of the magnetic structure;
g) means for separate and alternative or combined adjustment, particularly of the Field of Viewxe2x80x94or FOVxe2x80x94size and/or of image definition and/or image contrast and/or signal-to-noise ratio.
The means for separate and alternative or combined adjustment, particularly of the Field of View (FOV) size and/or of image definition and/or image contrast and/or signal-to-noise ratio, are continuous selectors or step-operated selectors whereby the parameters relating to the type of scanning sequences and/or of image processing procedures or to the combination of scanning sequences and/or of image processing procedures are adjusted, with reference to the effect on definition, contrast and signal-to-noise ratio.
In this case, there may be provided memories for storage of several different basic scanning sequences and/or processing procedures or combinations thereof or several different predetermined scanning sequences and/or processing procedures and relating to different Fields of View and/or to definition and/or contrast and/or signal-to-noise ratio qualities, the controls being arranged to adjust the parameters for execution of the different basic scanning sequences or processing procedures and/or to generate combinations of different scanning sequences and/or different processing procedures or to select combinations of scanning sequences or processing procedures or particular scanning sequences or processing procedures, which are prefixed and associated to the different desired Field of View size and/or definition and/or contrast and/or signal-to-noise ratio variations. Advantageously, the invention provides means allowing the user to control and select the sequences and/or the combinations of sequences which are identified, rather than by the typology of the sequences or combination of sequences in use, by the effect thereof on the displayed image, and especially on contrast and/or definition and/or Field of View size and/or signal-to-noise ratio and/or repetition of frames, i.e. the so-called frame rate.
Hence, the user has control capabilities oriented on the result to be obtained, i.e. a more detailed image, an image having a larger or smaller Field of View, an image with a better contrast, etc., without having to deal with complex technical problems relating to the individual sequences and to the parameters thereof, which are beyond the specific skills of the user, especially in the diagnostic art.
In accordance with an additional characteristic, the apparatus further includes:
h) means for measuring displacement direction/s and distances;
i) means for relating resonance signals to the relevant parameters of relative positioning of the body under examination and of the magnetic structure;
j) means for storing the different images and their respective position parameters;
k) processing means for combining images;
The processing means may include means for recognition of overlapping areas of the individual images of the limited partial regions and for combination thereof into the overall image so that said overlapping areas can be properly positioned.
Moreover, there may be provided correction or approximation algorithms which correct the shared portions between images so that an overall image with no substantial deformation is obtained.
The means for relative displacement of the body and of the magnetic structure may be of any type and may allow displacement, along polar Cartesian coordinates or the like, depending on the geometry of the magnetic structure, on the imaging volume and/or on the body to be examined.
Advantageously, in combination with the magnetic structure, there are provided means for supporting the body under examination or a part thereof, which are movable relative to the magnetic structure through the different displacement areas.
Alternatively, said movable support means are associated or borne by the receiving coil.
Particularly, in small magnetic structures like those for the apparatuses dedicated to the examination of body parts, of limited anatomical regions or limbs, the invention provides that the receiving coil is integrated in the magnetic structure and is not, as conventionally provided, a removable element.
This characteristic provides considerable construction advantages, by avoiding the presence of movable i.e. displaceable supports, associated to the receiving coil. In this case, the body part may be simply held up by supports of the overhanging type, projecting in the cavity of the magnetic structure and separated therefrom, for instance extensions of patient supporting chairs, seats or tables.
This arrangement may be implemented without affecting image quality, in said apparatuses with small magnetic structures, thanks to the fact that the imaging cavity is smaller and that the walls thereof which support the receiving coil are sufficiently close to the body or to the body part under examination.
Visual means for indicating the slice being scanned may be also provided, such as light pointers directed towards the body under examination in a position coinciding with the selected scan slice.
The above description clearly shows the simplicity of implementation of the method according to the invention. The necessary construction arrangements are simple and inexpensive. Especially as regards processing, no considerable changes are required in hardware, with the only need to load data processing, correlation and combination software therein. The apparatus for implementing the method does not require high-cost processing equipment, neither for speed nor for performance since, by using Fields of View whose size is limited to the size of the detail of interest of the body under examination, the amount of information to be processed even when complex sequences are used to obtain high quality images is still relatively limited.
Further improvements of the invention will form the subject of the subclaims.